Enhanced Photothermal Property of NDI-Based Conjugated Polymers by Copolymerization with a Thiadiazolobenzotriazole Unit.

Solar steam generation (SSG) is a promising photothermal technology to solve the global water storage issue. The potential of π-conjugated polymers as photothermal materials is significant, because their absorption range can be customized through molecular design. In this study, naphthalenediimide (NDI) and thiadiazolobenzotriazole (TBZ) were employed as bifunctional monomers to produce conjugated polymers. There are two types of polymers, and . is based on NDI, while is a copolymer of NDI and TBZ in a ratio of 9:1. Both polymers had high molecular weights and sufficient thermal stability. UV-vis-near-infrared (NIR) absorption spectra revealed that both polymers have large extinction coefficients ascribed to the NDI and TBZ chromophores. Notably, the absorption spectrum of exhibited a significant red shift compared to , resulting in a narrow optical bandgap and absorption in the NIR range. This result suggested that has a higher light absorption than . Photoluminescence (PL) spectra were measured to elucidate the conversion of the absorbed light into thermal energy. It was found that has a reduced fluorescence quantum yield as a result of the TBZ unit, signifying a proficient conversion of the photothermal energy. Based on the results, it appears that the film has a greater photothermal property compared to that of the film. The surface temperature of the film reached approximately 50 °C under the investigated conditions. In addition, copolymer exhibited an impressive SSG efficiency of 72.4% under 1 sun (1000 W/m) irradiation. All the results suggested that narrow bandgap conjugated polymers containing the TBZ unit are highly effective materials for achieving optimal performance in SSGs.